The medical field sees an increasingly widespread use of devices based on MEMS resonators that can withstand difficult conditions and operate as radio frequency identification (RFID) memories, in which the resonators are activated by the magnetic field generated by the current flowing in an antenna.
For this purpose, the antenna should meet some requirements, such as having an inductance value on the order of microHenries (μH), a small size, and a low cost. It has already been suggested to make the antenna on a BGA/LGA (Ball Grid Array/Land Grid Array) substrate. These substrates are formed by a plurality of overlaid conductive tracks (generally of copper, each formed in a conductive layer), and insulated from each other by insulating material layers. Holes, referred to as “vias”, allow electric contact through different insulating layers of the substrate. The electric contact in the holes is obtained by the metallization of the inner surface of the holes, obtained by a process of electrochemical plating or by applying a conductive material layer and by screening and a subsequent high temperature baking. Another method to produce the electric contact through the holes consists in totally filling the latter with an adhesive charged with conductive particles by screening and baking, or by injection and baking and baking the conductive adhesive. The holes mutually couple the conductive tracks so as to form a plurality of conductive paths. In this case, the antenna for the memories or other RFID device may be produced on one of the main surfaces of the BGA/LGA substrate, for example as a miniaturized loop antenna, formed by a track of copper or other conducting material.
This implementation, however, allows achieving only low values of inductance (a few nanoHenries), while, as indicated above, the application as an antenna for a RFID system may require values of about three orders of magnitude higher.